Dongki Min

Tunable two-dimensional acoustic meta-structure composed of funnel-shaped unit cells with multi-band negative acoustic property

This paper presents a two-dimensional heat-exhaust and sound-proof acoustic meta-structure exhibiting tunable multi-band negative effective mass density. The meta-structure was composed of periodic funnel-shaped units in a square lattice. Each unit cell operates simultaneously as a Helmholtz resonator (HR) and an extended pipe chamber resonator (EPCR), leading to a negative effective mass density creating bandgaps for incident sound energy dissipation without transmission. This structure allowed large heat-flow through the cross-sectional area of the extended pipe since the resonance was generated by acoustic elements without using solid membranes. The pipes were horizontally directed to a flow source to enable small flow resistance for cooling. Measurements of the sound transmission were performed using a two-load, four-microphone method for a unit cell and small reverberation chamber for two-dimensional panel to characterize the acoustic performance. The effective mass density showed significant frequen...

Control of a PWM converter in different input voltage conditions

A control scheme of a three-phase voltage-source PWM power converter available for the condition of different input voltages is proposed in this paper. From the steady-state model of the power converter, the effect caused by the input voltage on the control of the input current is analyzed from the point of the linear controllability of the input current. Sufficient voltage level of the DC link to control the input current linearly is then analytically determined. The effect of the different input voltage on the DC link voltage control with a fixed-gain controller is analyzed and a simple but effective online gain tuning algorithm is proposed to obtain the optimal regulation of the link voltage. In conclusion, the validity of the proposed control scheme is proved by the experimental results.

Active control of exhaust noise using an air horn

The noise generated by internal combustion engine is reduced by passive muffling systems. For the passive muffling system, its performance significantly degrades especially when multiple low frequency tonal components exist in the flow. The tonal components occur from explosion process of the engine, and radiates as a monopole from the outlet. In this study, active noise cancelation using FxLMS algorithm is proposed to reduce the exhaust engine noise. Air-horn which is capable of being operated at high temperatures is proposed for cancelation of the radiated noise. The vibration input the diaphragm of the air-horn allowed the active control of the frequency and phase of the radiated sound. The FxLMS algorithm was used to actively control the sound radiation from the air-horn to achieve cancelation of the noise from the muffler. The sound radiation from the air-horn induces dipole-like noise radiation from the exhaust system, and significantly reduced the radiated sound power.

Artificial Engine Sound Synthesis Method for Modification of the Acoustic Characteristics of Electric Vehicles

Sound radiation from electric motor-driven vehicles is negligibly small compared to sound radiation from internal combustion engine automobiles. When running on a local road, an artificial sound is required as a warning signal for the safety of pedestrians. In this study, an engine sound was synthesized by combining artificial mechanical and combustion sounds. The mechanical sounds were made by summing harmonic components representing sounds from rotating engine cranks. The harmonic components, including not only magnitude but also phase due to frequency, were obtained by the numerical integration method. The combustion noise was simulated by random sounds with similar spectral characteristics to the measured value and its amplitude was synchronized by the rotating speed. Important parameters essential for the synthesized sound to be evaluated as radiation from actual engines were proposed. This approach enabled playing of sounds for arbitrary engines. The synthesized engine sounds were evaluated for recognizability of vehicle approach and sound impression through auditory experiments.

Axial force measurement for U-bolt using wave propagation speed monitoring

It is important to inspect clamped parts of a produced vehicle in the manufacturing process non-destructively. In this study, a measuring device of axial force is suggested by utilizing relations between wave propagation characteristics of a beam and a clamping force. Experiments were carried out to determine characteristics of vibration signals for the structure designed for inspection. Neodymium magnet was utilized as the components of the device to attach to the joint without any damage to the products. A rectangular parallelepiped magnet was used to detect the change in the wave propagation speeds in the structure. The change in the wave propagation through the bolt was measured and used in estimation of the axial force. The correlation coefficient was calculated to quantify the change in the wave propagation characteristics. The coefficient was utilized to determine the axial force of the bolted joint by the experiment. The proposed method was applied to actual U-bolts to validate the methodology.

Measurement and analysis of squeal noise of vehicle wiper system

The squeal noise occurring from rubber and hard surface is an important issue, especially for vehicle wiper system. The noise is generated at relatively high frequency ranges, and its generation mechanism during the wiper operation is not straight forward to understand. In this study, experimental setup to generate the squeal noise in a consistent manner is designed using a rotating glass table. The wiper is located on the glass table in compressed status so that to simulate the wiper in the actual operating system. While the glass was rotating with constant velocity, the windshield washer fluid was sprayed on the glass. Under the normal constant velocity of the wiper operation, the condition (temperature, moisture contents, velocity) of continuous occurrence of the squeal noise was identified. The friction coefficient was measured, and its impact on the squeal noise generation was identified. The influence of various parameters such as the compression ratio, temperature, types of the wiper, angle of wipe...